Isomerization through Cleavage and Recombination of Imidazolide Linkage in the Condensed Tricyclic System Related to Hypermodified Bases of Phenylalanine Transfer Ribonucleic Acids.
作者:Taisuke ITAYA、Tae KANAI、Shin-ichi OHYAMA、Yoshihisa SHIRASAKI、Nobuhiro MURAMOTO、Yukiko ONO
DOI:10.1248/cpb.47.1464
日期:——
1-Benzyl-4, 6-dimethyl-4, 9-dihydro-1H-imidazo[1, 2-α]purin-9-one bearing an alkyl, a 1-alkenyl, a hydroxymethyl, a methoxymethyl, or a formyl group at the 7-position (3a-e) underwent rearrangement through fission and recyclization of the pyrimidine ring, attaining equilibrium with the corresponding positional isomer 4a-e in MeONa-MeOH at 25°C, whereas 7-methoxycarbonyl and 7-halogeno compounds 3f-i were irreversibly converted into the rearranged products 4f-i under identical conditions. The position of equilibrium appears to be affected by the electronic factor of the substitutent rather than the steric one. The pseudo-first-order rate constants measured for the reactions of 3a, b, d, f-i suggest that the reaction is accelerated by the electron-withdrawing substituent. However, the reactions of this series of compounds do not always obey the Hammett equation.On the other hand, a linear free energy relationship (ρ=+3.2) was observed for the rates of rearrangement of a 6-demethyl series of compounds 9a, b, d, f, g, when σ0P values were employed. The deviations from this relationship for the reactions with the 7-hydroxymethyl compound 9c and the 7-carbamoyl compound 9e are explicable in terms of the accelerative effect through intramolecular hydrogen bonding with the carbonyl oxygen at the 9-position.
1-苄基-4, 6-二甲基-4, 9-脱氢-1H-咪唑[1, 2-α]嘌呤-9-酮在7位具有烷基、1-烯基、羟甲基、甲氧基甲基或甲酰基的化合物(3a-e)通过嘧啶环的裂解和重环化发生重排,在25°C下在MeONa-MeOH中达到与相应位置异构体4a-e的平衡,而7-甲氧基羧基和7-卤代化合物3f-i在相同条件下被不可逆地转化为重排产物4f-i。平衡位置似乎受到取代基电子因素的影响,而非空间因素。对化合物3a、b、d、f-i反应测得的伪一级速率常数表明,该反应因电子吸引取代基而加速。然而,这一系列化合物的反应并不总是遵循Hammett方程。另一方面,对于6-去甲基类化合物9a、b、d、f、g的重排速率,采用σ0P值时观察到线性自由能关系(ρ=+3.2)。对于与7-羟甲基化合物9c和7-氨基甲酰化合物9e的反应,这一关系的偏离可以通过与9位羰基氧的分子内氢键相互作用的加速效应来解释。